New modes of crystal engineering with nitrogen-oxygen and sulfur-oxygen functional groups

Several N-O and S-O-based electron-rich molecules have been synthesized and tested for capability of forming electron donor-acceptor complexes (EDA) with electron-deficient molecules both in solution and in the solid state. In solution, the donating ability of each donor was evaluated in terms of its oxidation potential, charge-transfer absorption wavelength, and complex-formation constant. For solid-state study, X-ray single crystal structures of EDA cocrystals were obtained and analyzed in terms of intermolecular contact distances, frontier molecular orbital (FMO) overlap, stoichiometry and connectivity of donor to acceptor, and geometry of the donor-acceptor (DA) interactions. S-O based donors were found to form EDA complexes with 1,2,4,5-tetracyanobenzene (TCNB) of similar geometry to those known for N-O based donors with tetracyanoethylene (TONE) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The geometries of EDA complexes studied can be interpreted in terms of FMO overlap and coulombic interactions. Pseudoaxial SO bonds appear to more efficiently form close contacts with acceptors than the more sterically hindered pseudoequatorial SO bonds of trans-thianthrene-5,10-dioxide. The donor molecule in phenothiazine-S,S-dioxide/TCNB complex unexpectedly adopts a planar structure. The conformation change suggests the adaptive binding nature of the EDA bonding. The consequence of hydrogen bonds and ionic interactions on the molecular packing of nitroxyl radicals were also investigated to gain insight into magneto structural correlations in molecular (paramagnetic) solids.